Gas phase polymerizations for the production of polyolefins are typically carried out in fluidized bed reactors. The polymerization reaction is an exothermic process, making it important in the overall process to remove the generated heat of reaction to keep the operating temperature of the reactor below crucial temperatures such as the resin and catalyst degradation temperatures as well as temperatures which cause polymers to fuse and plug the reactor. Therefore, the amount of polymer that can be produced in a fluidized bed reactor of a given size in a specified time period is directly related to the amount of heat that can be withdrawn from the fluidized bed. Alternatively stated, the rate at which the heat of the polymerization reaction can be removed is a capacity limitation of gas phase polymerization reactors.
Operating a gas phase polymerization reactor in a condensation mode, where a recycle gas stream is cooled to a temperature below the dew point of the recycle gas stream to form a mixture comprising a liquid phase and a gas phase, is a typical way to remove the heat of the polymerization reaction. This mixture comprising the liquid phase and the gas phase is recycled back to the reactor and injected into the fluidized bed.
In a process utilizing condensing mode operation, the gas-liquid ratio should be maintained at a level sufficiently high to keep the liquid phase entrained in the gas phase of the recycle stream fed to the reactor. However, an excessive amount of condensation can lead to liquid pooling in the bottom head of the reactor. This liquid pooling can lead to fluidized bed instability problems. Furthermore, in a process utilizing condensing mode operation at high levels of condensation, undesirably high levels of liquid phase can exist in the lower sections of the fluidized bed. This can lead to liquid entrainment out of the reactor along with the product during a product discharge event. This liquid entrainment can reduce the fill efficiency of the product discharge system and increase the load on downstream vapor recovery systems. Also, if the condensed liquid is separated from the recycle gas and fed into the reactor through nozzles, the capacity of the liquid pump and liquid injection nozzle may become a limitation. During the summer months, the warmer ambient temperatures can adversely affect production rates by making it more difficult to cool the recycle to a temperature below the dew point of the recycle if an upper limit on the condensable component concentration is established. Additionally, increasing the concentration of the condensable component, e.g., isopentane, can increase the molecular weight of the recycle gas, thus increasing the energy consumption of the recycle gas compressor. Furthermore, high levels of isopentane may cause undue resin stickiness. Therefore, there is a need in the polymer industry for alternative methods of operation to increase production rates from polymerization reactions other than by continuing to increase the amount of condensable component in the recycle gas, especially during the warmer months of the year, which may improve overall process economics at other times of the year as well.